Miniature crossed coil gauge having an active flux ring

ABSTRACT

A miniature crossed coil gauge is provided having an oriented rare earth magnet fixedly mounted on a rotary shaft for supplying a magnetic flux directed transverse to the shaft axis. The gauge housing provided by an upper and lower bobbin surrounds the magnet and supports the rotary shaft. A first and second coil are wound about the gauge housing in a crossed coil orientation encircling the magnet. A flux ring is disposed around the first and second coils axially aligned with respect to the magnet. The magnet has sufficient strength and the flux ring is mounted in sufficiently close proximity to the first and second coils to redistribute the flux supplied by the magnet so that the magnetic flux measured at the surface of the coils is substantially increased relative to the magnetic flux density measured with the flux ring removed. This gauge construction facilitates the fabrication of gauges so small that they can be directly mounted on printed circuit boards using conventional electronic component mounting equipment.

RELATED CASES

This is a continuation of Ser. No. 08/968,773 filed Nov. 10, 1997, nowU.S. Pat. No. 6,046,583, which is a continuation of Ser. No. 08/548,584filed Oct. 26, 1995, now U.S. Pat. No. 5,686,832, which is acontinuation-in-part of Ser. No. 08/538,378 filed Oct. 3, 1995, nowabandoned, which is a continuation of Ser. No. 08/061,954 filed May 17,1993, now abandoned.

TECHNICAL FIELD

The field of the invention relates to an improved micro miniature analogmeter movement for an air core crossed coil type gauge.

BACKGROUND OF THE INVENTION

Analog instrumentation remains the most widely used and preferred methodof displaying automobile data to the driver. This is due to their simplefunction and ability to be adapted to many different styles. Evenexpensive automobile models that essentially have an unlimitedinstrumentation budget commonly choose analog gauges. Futurerequirements for instruments will unquestionably include analog gauges.A need is seen for new technology to help control the rising costs ofinstrument clusters by enabling a simpler more flexible instrumentcluster design concept.

Most current production automotive analog gauges utilize either aircore, D'arsonval, or bi-metal gauge technology.

One main problem with current air core gauges is their size and bulk.Mechanical complexity in the instrument housing, face plate andconnections are required to mount and constrain the gauge mechanisms.The instrument housing has evolved into a very complex part thatrequires tooling with long lead times and high cost. This high toolingcost is further magnified when “late changes” are required after thedesign is released for production. Automation of the assembly processrequires specially designed equipment for each type of instrumentcluster produced. The continual capital investment in this manufacturingequipment each time a new model is introduced drives the instrumentationcosts even higher.

The new micro-miniature gauge of the present invention does not requirethe complex mounting methods and housing complexity that today's largergauges need. The new gauge is directly mounted to the printed circuitboard which becomes the support structure for the entire instrumentcluster.

The new micro-miniature gauge becomes an enabling technology forflexible instrument cluster designs, miniature telltale modules, andanalog projection HUDs (heads up display).

Air-Core or Cross-Coil gauges have been used in automotive instrumentssince they were first invented in 1965 (see U.S. Pat. No. 3,168,689). Asshown in FIG. 1, a prior art crossed coil gauge 10 as shown utilizes atwo pole radially charged cylindrical magnet 12 attached to a coaxialshaft 14. The magnet rotates with the shaft in response to a resultantmagnetic flux vector. This flux vector is generated by two coils 16wound one over the other encircling the magnet. The coils are“air-core”, i.e., they contain no iron, and the first coil axis isperpendicular to the second coil. The coils are surrounded by an ironring or an iron can 18 to provide shielding from other external magneticfields. The magnet is housed in a plastic bobbin 20 that serves asbearing, damping fluid container, coil bobbin, iron can holder and gaugemounting means. Silicone damping fluid 22 fills the fluid containercavity formed in the bobbin and restrains magnet rotation. Gauge 10 hasa flat upper mounting surface 24 and a series of electrical terminals 26on the opposite surface.

Over the years several refinements have been proposed for aircoregauges. Most recently, U.S. Pat. No. 4,760,333 shows a novel way toprovide a magnetic shield utilizing a wound strip of amorphous metalalloy. This replaces the conventional iron can or ring to providemagnetic shielding. U.S. Pat. No. 4,827,210 shows a modification to theplastic bobbin to facilitate equal coil winding lengths to produce amore linear gauge. U.S. Pat. No. 4,992,726 shows woven (interlaced) coilwindings to produce a more linear gauge. U.S. Pat. No. 5,017,862 showsimproved bearing design and bobbin structure. U.S. Pat. No. 5,038,099teaches a radial air-core gauge design that balances the coil windingsand produces a thinner gauge. U.S. Pat. No. 5,095,266 shows a method ofcontaining viscous damping fluid for more uniform damping of the gauge.

Even though many incremental improvements have been shown in theliterature, several problems remain with current production air-coregauges. The first problem is size. Most air core gauges are from ¾ to 1¼inches in diameter and ½ to 1 inch long. They typically require complexmounting methods involving various screws, clips and plastic moldedparts. Special machines and processes are often designed to assemble thegauges into instrument clusters. The gauge thickness can control theminimum thickness of the instrument cluster and a larger gauge diametermakes backlighting the instrument dials difficult. Complex plastic lightpipes are often used to light gauge dials and pointers to bring lightfrom behind and around to the front of the gauge. Automated gaugeassembly directly to electronic circuit board assemblies is desirablebut difficult with current large air-core gauge designs.

A second unresolved problem involves the use of a liquid damping fluidin the air-core gauge. This is most commonly a silicon fluid of theappropriate viscosity. A messy production process is required to depositthe fluid inside the gauge bobbin assembly. Migration of the fluid oftenoccurs while the gauge is being transferred or stored or in actual use,resulting in lesser or sometimes no damping of the gauge after a periodof usage. Various attempts have been made to eliminate problems withviscous fluid damping. U.S. Pat. No. 5,095,266 shows a recent attempt.

Magnetic eddy current damping has been used in devices other thanair-core gauges to eliminate a viscous fluid. One of the most commondevices utilizing eddy current damping is a watt-hour meter used byelectrical utilities. The use of eddy current damping in these devicesis described in U.S. Pat. Nos. 4,238,729 and 4,238,730. The eddy currentdamping provides a continuous drag torque on a rotating electricallyconductive disk or cylinder which balances the disk drive torqueproviding a disk speed proportional to the rate of power consumption. Itis generally not used as a transient torque vibration damper in thisapplication. U.S. Pat. No. 3,786,685 shows a copper cup-shaped ring thatrotates in a magnetic field to dampen transient motion in a gyro. U.S.Pat. No. 3,983,478 discloses a moving coil instrument with copper ringsfor eddy current damping. These rings are welded to the moving coil andcut through a magnetic field to slow their rate of rotation. These eddycurrent damping devices all depend on high magnetic flux densitiesgenerated by using small air gaps and flux concentrating iron polepieces or multiple magnets. They are generally bulky and heavy becauseof the high flux requirements for damping.

It has been observed that all current production air core gauges utilizethe iron can or ring to provide magnetic shielding. Because of thedesign geometry and the magnets used, the can has little effect on theflux linking the gauge coils. Thus, the iron can or ring has littleeffect on the gauge torque which is primarily limited by the magnetmaterials, the gauge geometry and the number of amp-turns produced bythe coil windings.

Air-core gauges have been used in various “telltale” designs to rotateicon masks in front of a light to indicate a failure in one of a numberof automotive systems (see U.S. Pat. No. 3,660,814). In theseapplications a problem exists that no detente action is provided in thegauge to “hold” a position. Therefore, continuous power is generallyrequired. A need exists to provide a method of modifying the magneticreluctance of the air core gauge to provide unique torquecharacteristics in the gauge for detente action or to linearize thegauge.

In summary, several current air-core gauge problems have beenidentified. These problems are: a.) large gauge size; b.) unreliableliquid damping; c.) inefficient magnetic flux utilization; and d.)constant magnetic reluctance.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a novel miniature aircore gauge that can be assembled to an electronic circuit substrate asthough it were a standard electronic component. Using this new enablingtechnology, a flexible circuit board assembly process can be used tomanufacture a variety of automotive instrument clusters utilizingcommonly available circuit board equipment with high flexibility.

It is an object of the invention to reduce the size of the gauge to assmall as possible to enable assembly of the gauge to electronic circuitboards by common electronic component assembly equipment.

It is a further object to assemble the gauge by electrical connectionsfrom either end of the gauge.

Another object of the invention is to utilize magnetic damping orferrofluids replacing the conventional silicon fluid viscous dampingmeans.

Still another object of the invention is to provide means for greatlyincreased flux utilization to both increase gauge torque and makemagnetic damping possible.

Finally, another object of the invention is to make a variablereluctance flux ring to enable gauge torque variation as a function ofmagnet angular position to linearize the gauge or make stepping actionof the gauge possible.

It has been discovered that prior art air-core gauges use large diameterceramic or ALNICO magnets. A few use unoriented plastic molded rareearth magnets but are more costly. It has also been discovered that inthese prior art gauges the amount of magnet flux threading the coils isnot significantly increased by the addition of the iron can. In theseapplications the flux ring or flux can serves primarily as a magneticshield to prevent external fields from affecting gauge accuracy.

In the present invention, the combination of a small diameter flux ringof sufficient thickness will significantly effect the available magnetflux to the coils when used with high strength magnets. It has beendetermined that such a flux ring will cause the magnet flux toredistribute from the magnet poles in such a manner as to significantlyincrease the magnet flux encircling the gauge coils. This worksespecially well with the very high strength sintered neodymium magnets.The total magnetic flux of the magnet remains relatively constant whileits flux is redistributed by the flux ring. Thus, with the combinationof a small diameter flux ring and high strength orientated neodymiummagnet the ring becomes not only a magnetic shield but also asignificant influence on the flux available around the coils. In thepresent invention, high flux levels boost torque and provide theintensity necessary to make direct magnetic damping possible.

Further objects are implicit in the detailed description which followshereinafter (which is to be considered as exemplary of, but notspecifically limiting, the present invention) and said objects will beapparent to persons skilled in the art after a careful study of thedetailed description which follows.

For the purpose of clarifying the nature of the present invention, oneexemplary embodiment of the invention is illustrated in thehereinbelow-described figures of the accompanying drawings and isdescribed in detail hereinafter. It is to be taken as representative ofthe multiple embodiments of the invention which lie within the scope ofthe invention.

Accordingly, a crossed coil gauge as provided is a rotatable shaftoriented along a gauge shaft axis. Affixed to the shaft is a rare earthmagnet for generating magnetic flux transverse to the shaft axis. Themagnet is generally cylindrically shaped and has a radius which is about1.4 or less than the magnet axial length. A gauge housing is provided byan upper and lower bobbin disposed around the magnet and pivotallysupporting the shaft. A first and second coil is wound around the gaugehousing with the first coil wound generally perpendicular to the secondcoil and the first and second coils encircle the magnet in a crossedcoil manner. A flux ring is disposed around the first and second coilsand axial aligned with the magnet and shaft. The magnet has sufficientlyhigh strength and the flux ring being mounted in a sufficiently closeproximity to the first and second coils to distribute the magnetic fluxapplied by the magnet to actively increase gauge performance. Themagnetic flux density nearer the first and second coil surface beingsubstantially increased when the flux ring is present in comparison tothe magnetic flux measured with the flux ring off.

In a preferred embodiment of the invention, a damping ring is providedto generate eddy current damping for limiting magnet and shaftoscillations. The damping ring is a generally cylindrical non-ferrouselectrically conductive member positioned within the gauge housingbetween the upper and lower bobbins spaced from and surrounding themagnet. The damping ring is fixed relative to the housing and encircledby the first and second coils.

In a preferred embodiment of the invention, the flux ring is formed of amixture of iron and nickel utilizing a powder metal process to achieve athick cross-section part of very concentric and round geometry.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional view of a prior art air core gauge;

FIG. 2 is a perspective view showing an exemplary first embodiment ofone representative form of the invention;

FIG. 3 is a graph showing the step response in gauge position withmagnetic damping versus no damping for the first embodiment of theinvention;

FIG. 4a is an axial end view of a circular flux ring;

FIG. 4b is an axial end view of an oval flux ring;

FIG. 4c is an axial end view of a Tri-bump flux ring;

FIG. 5a is a graph illustrating uncompensated gauge error as a functionof gauge angle for a circular flux ring illustrated in FIG. 4a;

FIG. 5b is a graph illustrating gauge error as a function of gauge anglefor an oval flux ring illustrated in FIG. 4b;

FIG. 5c is a graph illustrating gauge error as a function of gauge anglefor a Tri-bump flux ring shown in FIG. 4c;

FIG. 6 is a block diagram showing how various positive features areenabled by the combination of three basic component technologies;

FIG. 7 is a perspective view of a circular cross-section flux ring;

FIG. 8 is a side elevational view of a second embodiment of theinvention;

FIG. 9 is a front end view taken along line 9—9 of FIG. 8;

FIG. 10 is a rear end view taken along line 10—10 of FIG. 8;

FIG. 11 is a cross-sectional side elevational view taken along line11—11 of FIG. 8;

FIG. 12 is an axial end sectional view taken along line 12—12 of FIG. 8;

FIG. 13 is an exploded side cross-sectional elevational view of thesecond embodiment of the invention shown in FIG. 8;

FIG. 14 is a cross-sectional side elevational view of the secondembodiment of the invention shown in FIG. 8 during the flux ringinstallation process;

FIG. 15 is an enlarged rear end view of the second embodiment of theinvention showing a detail of the wire connection;

FIG. 16 is a side elevation view of a third gauge embodiment of thepresent invention;

FIG. 17 is an enlarged portion of the third gauge embodiment;

FIG. 18 is a cross-sectional side elevational view of a fourthembodiment of the invention;

FIG. 19 is a perspective view of a bobbin spacer utilized in the fourthgauge embodiment;

FIG. 20 is a side elevational view of the second gauge embodiment in atypical instrument cluster application;

FIG. 21 is a side elevational view of the fourth gauge embodimentmounted in an automotive instrument cluster application; and

FIG. 22 is a cross-sectional side elevational view of a fifth gaugeembodiment.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 2, an exploded view of the major components of theminiature crossed coil gauge 30 is shown. The gauge housing is formed ofthree pieces; the upper bobbin 32, lower bobbin 34, and damping ring 36.These three pieces fit together to form a bearing support and closedcavity to house the shaft 38 and magnet 40 assembly. This assembly alsoprovides support for first coil 42 and second coil 44. Bearing 46 ispressed into lower bobbin 34 to provide a thrust bearing 46 for shaft38. Damping ring 36 is made of copper and provides a unique method ofdamping the gauge to remove oscillations in the rotation of the magnet40 and any externally connected pointers. Damping ring 36 can optionallybe made from aluminum. The flux from magnet 40 passes perpendicular tothe circumference of damping ring 36 relative motion between magnet 40and damping ring 36 induces an eddy current in the damping ring 36 thatprovides eddy current generated damping. This method replaces theconventional silicon damping fluid generally used in air core gauges.

Terminal pins 48 are molded or inserted into upper bobbin 32 and lowerbobbin 34 to provide front or rear mounting of the gauge to a printedcircuit board. Terminal pins 48 may also be shaped for surface mountingthe gauge to circuit board assemblies.

Flux ring 50 provides a closed flux return path for magnet 40 and firstcoil 42 and second coil 44. It has been found in this configuration thatthe magnet flux density near the magnet poles at the coils can beapproximately doubled by the addition of the flux ring 50. Flux ring 50also provides shielding from external magnetic fields to keep otherdevices from interfering with the gauge. Magnet 40 in this design is arare earth type magnet and can be non oriented or a high performanceoriented magnet. More effective eddy current damping can be achieved byusing a higher flux density magnet 40 that has an outwardly directedflux pattern.

Flux ring 50 can be made from low carbon steel but it is preferred touse a nickel alloy steel. It is important when using a higher strengthmagnet 50 to have sufficient flux ring 50 thickness to avoidmagnetically saturating flux ring 50 material and thus limiting gaugeperformance.

The following table provides a comparison of various characteristics ofthe first embodiment of the invention versus a prior art air-core gauge.

TABLE 1 Air-Core Gauge Comparisons First Prior Art Embodiment PreferredStandard Mini-Gauge Range Gauge Gauge Outside Diameter .540 inch .3-.61.005 inch Gauge Overall Height .400 inch .3-.6   .720 inch MaximumTorque* 2.00 g-cm. >1.5 1.25 g-cm. Magnet Type Sintered- Dense RarePlastic oriented Earth Molded Neodymium Neodymium Magnet Volume .0088in³ <.01 in³ .026 in³ Magnet Diameter .250 inch .15-.35 .530 inch MagnetRadius 0.125 inch   .075-.175 0.265 inch Magnet Length 0.179 .35-.74times 0.118 inch the diameter Peak Flux Density 5.14 >4.0 2.15 @ MagnetSurface Kilogauss Kilogauss Kilogauss Peak Flux Density 1.58 >1.0 450gauss @ Flux Ring Kilogauss Kilogauss Peak Flux Density 948 gauss <75%of 380 gauss @ Flux Ring (w/o ring) Flux w/ring Percentage Change in67% >30% 18% Peak Flux Density Caused by Ring *The maximum gauge torquesare compared at equal coil amp-turns to compare magnetic performance ofthe gauges.

It can be seen from this data that the mini-gauge is much smaller thanthe prior art. It is about one-half the diameter and about one-half aslong as the prior art. Yet, on an equal amp-turn basis the magnetics ofthe mini-gauge produces an output torque that is 60% higher than priorart. The prior art gauge chosen for comparison uses a state-of-the-artmolded neodymium-iron magnet that produces a relatively high flux levelwhen compared to other prior art air-core gauges that use either ceramicor alnico magnets. Therefore, other prior art aircore gauges would showyet lower torque performance when compared to the subject invention. Themagnet volume of the mini gauge is only ⅓ of the prior art. Because ofthe small size, the more expensive sintered-oriented neodymium ironmagnet can be utilized in the mini gauge but becomes cost prohibitive touse in the prior art gauge. Even though the torque of the mini gauge ishigher than the prior art the magnet is only ½ the diameter. The minigauge has 2.4 times the flux level on the magnet surface compared toprior art. The peak flux density into the flux ring is 3.5 times theprior art. The peak flux density near the coils without the flux ring isreduced to 948 gauss in the mini gauge and 380 gauss in the prior artgauge. Comparing the percentage change in peak flux density caused bythe flux ring shows that the mini gauge gets a much larger boost inavailable coil flux by the addition of the flux ring when compared toprior art (67% versus 18%).

Thus, the flux ring of the present invention becomes more than just ashield and plays a more important role in the magnetic circuit of thegauge and thus it's performance. The combination of compact size andhigh powered magnet gives a result much different than prior artair-core gauges and it's potential is not obvious without detailedanalysis of the magnetic materials and geometry involved.

The higher flux level attained at the coils in the mini gauge was foundto be primarily caused by the redistribution of the magnet flux near thecoils when the flux ring of sufficient length and thickness is added.The overall magnet flux is not appreciably increased, but ratherdirected outward further from the magnet by the addition of the ring.This was confirmed by search coil measurement of total flux in themagnet with and without the flux ring. Also, computer models of the minigauge cross-section confirms this test result.

FIG. 2 shows an electrically conductive cylindrical damping ring 36 usedin the present invention. It is only possible to consider magneticdamping in the air-core gauge when the magnet flux is very high. Table 1shows that the flux level of the mini gauge is 3.5 times that of theprior art when containing a plastic molded neodymium magnet. Other priorart gauges produce even lower flux levels pushing this ratio from3.5:1.0 as high as 7.0:1.0.

With reference to FIG. 7, the following expression defines magneticdamping torque as a function of several parameters, including the fluxdensity through the conductive damping ring.

T _(r) =K ₁ l ² B _(g) ² Vrwt/p*

where:

T_(r)=cylinder retarding or eddy current braking torque.

l=length of braking magnetic field parallel to the axis of the cylinder.

B_(g)=flux density in the air gap due to magnetic field from permanentmagnet normal to the surface of the cylinder.

V=ωr=tangential velocity of the rotating magnetic flux.

r=effective torque radius.

w=width of braking magnetic field perpendicular to axis of cylinder.

t=thickness of the cylinder.

p=electrical resistivity of cylinder.

K₁=proportionality constant.

* Basic Equation is from U.S. Pat. 4,238,729 adapted to a cylinder.

It can be seen that the damping torque is proportional to the square ofthis flux density and the square of the magnet length times the magnetradius. Since, the mini gauge has 3.5 to 7.0 times the flux level asprior art air-core gauges and the length increase offsets most of thediameter decrease, damping torque capability of the mini gauge isapproximately 6 to 24 times that of prior art air-core gauges. This factmakes magnetic damping a viable option in the new mini-gauge and leavesfluid damping the only real possibility in prior art air-core gauges.

The magnetic damping used in the mini gauge can be easily realized byputting a conductive material between the rotating magnet and the fluxring. As the magnet moves, a torque opposing it's motion is generated inthe damping ring and the magnet motion is stabilized under transienttorque conditions. The magnetic damping provides obvious advantages overliquid viscous damping currently used in prior art gauges. Filling thebobbin cavity with fluid is eliminated during production, the dampingcan not be lost in handling the gauge or during the use of the gauge.Finally, the entire gauge bobbin can be spun at high speed (because nofluid can be thrown out) to facilitate alternate methods of winding thegauge coils.

FIG. 3 shows a comparison of the gauge angular displacement in responseto a step input in drive torque. With no damping, the gauge will “ring”at low frequency as shown by curve 52. With magnetic damping, the“ringing” is significantly reduced as shown by curve 54.

As has been shown, unlike prior art air-core gauges the mini gauge fluxring is a significant factor in the magnetic circuit. Because it canchange flux levels at the coils by over 60%, it now can be considered asa variable reluctance device.

FIGS. 4a, 4 b, and 4 c illustrate three different flux rings 50, 52, and54 of different configurations starting with the standard circular fluxring 50 in FIG. 4a. FIGS. 5a, 5 b and 5 c provide graphs showing gaugeangular position error as a function of gauge angle for each flux ring.This error results from static torque variations as the gauge magnetrotates through 360 degrees. In the circular flux ring case, the gaugeerror curve 56 has four humps of about 3 to 5 degrees error caused bythe outside gauge coil being of a lesser influence on the magnet thanthe inside coil. This is because for an equal number of turns theoutside coil has higher resistance and is spaced further away from themagnet than the inside coil.

The oval flux ring 52 of FIGS. 4b and 5 b provides a method tocompensate the nonlinear result from the first case. The reluctance ofthe gauge magnetics can be adjusted by rotating the oval flux ringrelative to the coil bobbin in such a manner as to balance the unequalcoil torques. The reluctance torque offsets the torque produced by thecoil imbalance such that the gauge position error can be reduced asshown by curve 58 in FIG. 5b.

The third flux ring design shown in FIGS. 4c and 5 c is a tri-bumpdesign illustrating how the magnetic reluctance torque of the gauge canbe further modified to produce a simple stepping action from the gauge.In this case a tri-bump flux ring 54 is used. With a two-pole magnet,six torque bumps can be produced in the gauge as shown by curve 60 inFIG. 5c. The mini gauge magnet will preferentially align at sixdifferent locations providing a form of memory that will hold the gaugeposition similar to a stepper motor. Of course, many variations of thisidea can be envisioned producing different numbers of steps. Anymodification of the flux ring to change the reluctance as a function ofgauge angle could also be used (i.e., holes or thin walls).

FIG. 6 is a relational block diagram that shows how the uniquecombinations of the flux ring, conductive cylinder, and magnet provideenabling technology. This results in a very small new aircore gaugecalled the mini gauge. It has several unique features that enable manyother benefits as shown on this chart. These benefits all result fromthe small physical size of the gauge and the very high flux densitiesgenerated at the gauge coils. The flux ring itself becomes much morethan a shield against external magnetic fields and is an active elementin the system that can be modified to directly effect gaugecharacteristics.

FIGS. 8-15 illustrate a second embodiment of the invention earlierreferred to as crossed coil gauge 62. The second embodiment of theinvention is substantially similar to the first embodiment of theinvention shown by crossed coil gauge 30. Crossed coil gauge 62 is shownwith additional features that further improve its accuracy andrepeatability from gauge to gauge.

Crossed coil gauge 62 is shown assembled in FIGS. 8-10. Longitudinal andradial cross-section of crossed coil gauge 62 is shown in FIGS. 11 and12. For simplicity purposes, FIG. 13 shows an exploded cross-sectionalview of crossed coil gauge 62 without the first and second coils.

Gauge 62 is made up of upper and lower bobbin 64 and 66. Interposedbetween upper and lower bobbins 64 and 66 is a cylindrical damping ring68. Pivotally mounted on the upper and lower bobbins is gauge shaft 70which supports cylindrical magnet 72. A flux ring 74 is provided whichsurrounds the upper and lower bobbin assembly in axial alignment withmagnet 72. A pair of coils, first coil 75 and second coil 77, are woundabout the bobbin assembly prior to the installation of the flux ring.The wire ends of the wires forming first and second coils 75 and 77 areeach attached to one of four terminal pins 80.

In the embodiments of the invention illustrated in this application, thecrossed coil gauges are intended to be mounted directly upon a circuitboard without additional mounting devices or electrical connectors.While it should be appreciated that alternative forms of mounting can beutilized, the direct circuit board mounting is particularly advantageousdue to low installation costs which are now achievable due to the verylow mass of the present invention. Terminal pins 80 serve as theelectrical conductors coupling the first and second coils to the gaugedrive circuitry, and to physically mount the crossed coil gauge assemblyon to a circuit board without any additional electrical or supportingmeans. Again, due to the extremely small gauge axial height, the gaugecan be mounted on the front surface of the circuit board which wouldface the vehicle driver in an automobile instrument cluster application.

In the embodiment illustrated, the elongated portion of shaft 70projects from the gauge assembly from what is referred to as the “upper”end of the gauge, while terminal pins 80 extend from the gauge assembly“lower” or opposite axial end. of course, a wide variety of orientationsand configurations can be utilized, the illustrated configuration isbelieved to be particularly suitable for automobile instrument clusters.While the gauge in actual use can be oriented in any position, forreference purposes, the gauge shown in FIGS. 8 and 11, side elevationsshow the gauge in an orientation with shaft 70 vertical. With referenceto this orientation, bobbin 64 is referred to as the upper bobbin andbobbin 66 is referred to as the lower bobbin. The end of the gaugeassembly from which shaft 70 projects is the upper end, and the end ofthe gauge assembly from which terminal pins 80 project is the lower end.

Upper bobbin 64 is a molded plastic part having a central hub 76 with agenerally cylindrical bore formed therein for pivotally cooperating withshaft 70. Extending radially outward from central hub 76 is a generallydisk shaped flange 78 having a generally planar and lower surface 79. Atthe outer periphery of disk shaped flange 78, there are fourcircumferentially spaced apart bosses, each having an elongated centralaperture 84 for cooperating with terminal pin 80. Each boss 82 is alsoprovided with a flexible latching member 86 which is inwardly radiallybiasable to facilitate the installation of flux ring 74 thereover.Latching member 86 is provided with an upper ramp surface 88 forengaging flux ring 74 and a stepped shoulder 90 for capturing the fluxring and maintaining the flux ring at a specified axial orientationrelative to the gauge assembly. Preferably, upper bobbin 64 as well aslower bobbin 66 are injection molded using a nylon 66 material filledwith 20% Teflon. This material has exhibited excellent stability and lowfriction characteristics.

Lower bobbin 66 is similarly provided with a central hub 92, a generallydisk shaped radial flange 94, and four circumferentially spaced apartbosses 96. In the embodiment illustrated, central hub 94 is providedwith a blind cylindrical cavity having an integrally formed centralpivot point for pivotably cooperating with the lower end of shaft 70.The lower most external portion of hub 92 is formed by a downwardlyinwardly tapered frusto-conical pilot surface 98 which extends below theplane of mounting surface 100 to facilitate the precise orientation ofthe gauge assembly on a circuit board during an automatic assemblyprocess.

Flange 94 of lower bobbin 66 is provided with a generally planar uppersurface 102 for cooperation with damping ring 68. When assembled,damping ring 68 engages planar surface 79 on upper bobbin 64 and planarsurface 102 on lower bobbin 66 to define a sealed generally cylindricalmagnet cavity 104. Damping ring 68 serves to space upper and lowerbobbins apart as shown in FIG. 11.

Each boss 96 on lower bobbin 66 has a longitudinally extended bore 106formed therein. Bores 106 are sized to receive terminal pins 80 whichare press fit therein. Terminal pins 80 extend both above and belowbosses 96 as illustrated. The portion extending above the bosscooperates with bore 84 in the upper bobbin boss to rotationally alignthe boss and retain the upper and lower bobbins prior to coil winding.Terminal pins 80 also extend below bosses 96 to facilitate the surfacemounting of gauge assembly 62 on a printed circuit board.

Upper and lower bobbins 64 and 66 have a number of subtle designfeatures to facilitate assembly. Flange members 78 and 94 are generallycircular as indicated in FIGS. 9 and 10. The outside diameter of flangemembers 78 and 94 are preferably greater than damping ring 68 so thatcoils 75 and 77 which are wound about the upper and lower bobbinassembly do not contact damping ring 68. This minimizes the likelihoodof shorting the coil out on the damping ring. Another feature which aidsassembly is the construction of bosses 82 and 96. The radially innermostportion of bosses 82 and 96 extend inboard of the outer periphery offlange members 86 and 94 to prevent nesting of like parts when theseparts are located together in any container. This feature eliminates anentanglement problem during gauge assembly.

As illustrated in FIGS. 10, 11, 13 and 15 the lower most portion of eachof the bosses 96 is defined by three feet: fixed inboard foot 108, fixedoutboard foot 110, and flexible outboard foot 112. Two fixed feet 108and 110 are each provided with a generally planar flat molded surface114 and 116 for cooperation with planar mounting surface 100. Flexibleoutboard foot 112 is slightly shorter as illustrated in FIG. 13 in orderto enable the foot to freely flex radially inward without binding on thecircuit board. The upper surface 118 and 120 of feet 112 and 110 arerespectively spaced from the circuit board to provide a shoulder forcooperating with one axial end of flux ring 74. Flux ring 74 isprecisely centered concentrically relative to the axis of shaft 70 byflexible feet 112 formed on each of the four circumferentially spacedapart bosses 96. Flexible feet 112 have an engagement surface 122 whichis located at a radius relative to the shaft axis, slightly greater thanthe inner diameter of flux ring 74 so that each of the four feet willcontact and coaxially align the flux ring 74 with shaft 70. The upperaxial end of flux ring 74 is similarly coaxially aligned by latchingmembers 86.

Flux ring concentricity and roundness are important factors since, aspreviously described, Both affect gauge linearity and gauge-to-gaugerepeatability. The flexible mounting of the flux ring relative to thelower and upper bobbins enables the bobbin material to contract andexpand with changes in temperature without deforming flux ring 74 orpermanently deforming the bobbin.

Flux ring 74 is preferably formed of a 50/50 nickel/iron alloy having alow residual induction such as the material described in ASTM Standard A904 dated 1991. In order to facilitate manufacturing, it has been foundthat the use of a powdered metal forming process is ideally suited forfabricating flux ring 74. This process allows high material utilization,excellent roundness, low fabrication and tooling costs, and enables theformation of uniform thick walled parts. The selected nickel ironpowdered metal alloy is compacted to at least 7.0 g/cc and preferably atleast 7.15 g/cc to achieve what is referred to as a fully dense preform.Once centered the resulting flux ring will have a density of at least7.2 g/cc and preferably at least 7.4 g/cc.

Currently, the flux rings utilized with the prior art crossed coilgauges are formed using a drawing process. A typical flux ring of theprior art does not play a significant role in gauge performance, actingprimarily as a shield, therefore tolerances are relatively unimportant.Furthermore, the geometry of prior art ring may not even becircumferentially uniform. Flux ring 74 of the present invention iscylindrical in shape having a circumferentially uniform cross-section.The inside diameter of flux ring 74 is approximately ½ inch, withpreferably no more than 0.002 inches variation in roundness andpreferably, less than 0.001 inches variation. To achieve roundness, fluxring 74 is sized with a sizing die after the initial sintering process.After the flux ring is sized, it is resintered in order to anneal thering and eliminate any variances in magnetic properties within the fluxring. Flux ring 74 has an axial length of approximately 0.38 inches anda wall thickness of 0.045 inches. Flux ring 74 can vary in length,however, the length should not be less than the length of the magnet andthe wall thickness should be sufficient to avoid magnetic saturation.Wall thickness ideally is greater than 8% of the flux ring length andpreferably greater than 10% of the flux ring length.

As described with reference to the first gauge embodiment of flux ring74, it plays a significant roll in the magnetic circuit connecting thenorth and south poles of magnet 72. The influence of the flux ring canbe evaluated by measuring the flux using a hall probe sensor at thesurface of the coils adjacent the magnet pole with both the flux ring inplace with the flux ring removed. Preferably, the flux ring of thepresent invention will cause the peak flux measured at the coil surfacewith the probe sensor to increase at least 30% preferably over 50%.

In addition to making flux ring 74 utilizing a powder metal formingprocess, the flux ring can be formed using alternative methods whichachieve a fully dense metal flux ring. The term “fully dense” is usedherein refers to formed powder metal parts having 90% of the density ofthe pure metal alloy and preferably over 92% of the pure metal alloydensity. These densities are obtainable in using the properly selectedpowder metals compressed to a relatively high level of compression.Higher densities approaching 100% can be achieved using a metalextrusion or metal injection molding process. The added increase in fluxring density however, may not be worth the added expense. Although it isbelieved that extruded or metal injection molded flux rings work quitesatisfactorily, at the present time, flux rings formed of powder metalwhich are fully dense have a cost advantage. Of course, the flux ringcan be formed using a conventional stamping process from a metal blank.However, due to the heavy wall thickness need for the present flux ringand the relatively small size, stamping is not believed to be economicaldue to the very high scrap rate and high cost of the raw material.

As previously described, magnet 72 is formed of a very high strengthoriented rare earth magnet material. In particular, fully dense powdermetal NdFeB and SmCo magnets perform satisfactorily. Selected magnetmaterials have an energy product over 25 MGO and preferably 28-60 MGO.It is believed that further advances in magnet technology which shouldincrease magnet strength and reduce magnet cost will further increasethe cost benefit comparison of the present invention versus gauges ofthe prior art construction.

The magnet 72 of the present invention has a diameter of 0.250 and anaxial length 0.165. Ideally, the axial length of the magnet will exceedthe magnet's radius. Most preferably, the magnet's axial length will beapproximately 0.70 to 1.50 times the magnet's radius. Magnet 72 ismagnetized in a manner so as to have two magnetic poles orientedradially 180° apart. Ideally, the magnet has a minimum strength of 3.0Kilogauss, and most preferably, at least 4.0 Kilogauss measured ateither pole on the magnet surface.

Magnet 72 is concentrically affixed to shaft 70 utilizing a conventionalmetal injection molding process using lead/tin alloy, a zinc alloy, asilver/tin alloy, or an antimony/tin alloy. When magnet 72 is NdFeB, itis preferably nickel coated using an electroplate process in order tominimize corrosion. To achieve good pull out strength, shaft 70 isknurled in the region of the magnet with a diamond or spiral knurlpattern. Magnet 72 has a hole formed there with annular recesses at eachend to allow injection metal to be flush with the end faces of themagnet, thereby maximizing shaft push out strength. This method keepsthe metal section material below the magnet face allowing room in thebobbin cavity for additional magnet material.

Damping ring 68 is designed to have an axial length greater than themagnet and an inside diameter greater than the magnet's diameter todefine an enclosed cavity for the magnet to rotate within. Damping ringwall thickness effects the amount of damping. Wall thickness is between0.018 to 0.040 inches, preferably above 0.020 inches. To avoid shortingof the coils, the damping ring is coated with an insulator such asParalene™. Damping ring 68 is formed of annealed electrically pure OFHCcopper (oxygen-free high conductivity), however, other non-magneticelectrically conductive materials, such as aluminum can be used providedthey exhibit the requisite eddy current damping characteristics.

Once the magnet, shaft, damping ring and bobbins are assembled, thesubassembly is placed in a coil winder for winding first and secondcoils 75 and 77. Coils 75 and 77 are wound using conventional coppermagnet wire. Wire gauge is preferably 38 to 50 gauge, and mostpreferably 44 gauge. Ideally, approximately 500-1000 turns, andpreferably about 800 turns, are wound in each coil. As noted withreference to FIG. 5a, a non-linearity results from the increase inresistance in the outer most of the two coils due to the added wirelength for a given number of turns. In order to maintain the resistanceand the number of amp turns equal, a compensation resistor or additionalback-wound turns can be added to the innermost coil. Back windingincreases the inner coil resistance while maintaining the proper netnumber of amp turns.

An alternative method of winding the coils to achieve equal amp turnsand resistance involves varying wire winding tension. The innermost coilis wound in a conventional manner except that the tension on the wireduring winding is adjusted to stretch the wire and increase theresistance. The outer coil is then wound at the normal tension. Smallchanges in wire tension when using fine wire can cause significantchanges in coil resistance. This method is made practical by the smallsize of the coils resulting from the relatively few number of turns andthe thin wire used in the present gauge. There is less than 10%difference in resistance between the coils when the coils are formed of800 turns of 44 gauge wire with normal tensioning. This relatively smalldifference in resistance is very significant compared to the prior artwhich uses a much heavier wire and 2-4 times the number of turns. Analternative means for balancing the resistance and the net number of ampturns is the use of a calibration resistor in series with the first coilto appropriately increase its resistance, thus reducing its amp turns.

Once the coils are wound, the wire ends forming the coils are terminatedon terminal pins 80. Feet 108, 110 and 112 are spaced about the terminalpins a distance sufficient to form a pocket into which the wound wireends may recess enabling the gauge assembly to be flush mounted on to acircuit board. Once the coils are wound and terminated, flux ring 74 canbe axially installed over the upper bobbin as shown in FIG. 14. Latchmembers 86 flex radially inward as illustrated enabling the flux ring topass thereover.

An enlarged detail of the foot design of the lower bobbin is shown inFIG. 15. Fixed inner foot 108 has a smooth round contour to enable wire124 to wrap thereabout providing a strain relief for the wire.

A third gauge embodiment 130 is illustrated in FIG. 16. Gauge 130differs from gauge 62 described previously in three ways: (i) theaddition of magnetic biasing to axially orient the magnet and shaftrelative to the bobbin; (ii) the absence of a damping ring and the useof a ferrofluid magnetic damping fluid; and (iii) the method of affixingthe magnet and shaft.

Lower bobbin 132 is provided with a thrust disk 134 which serves tomagnetically cooperate with the magnet 136 and associated shaft 138.Thrust disk 134 is a flat washer made of a fully annealed nickel/ironalloy which is approximately 0.015 inches thick having a 0.160 outsidediameter and a 0.080 inside diameter. Thrust disk 134 serves to biasmagnet 136 downward to maintain the lower end of shaft 138 securelyengaged within the lower bobbin 132. This construction enables the gaugeto be used in any angular orientation and in high vibration environmentswhile preventing the magnet from contacting the upper bobbin.

Gauge 130 does not utilize eddy current damping. Rather, lower bobbin132 and upper bobbin 135 sealingly engage one another to define anenclosed magnet cavity 140. Once the gauge is substantially complete andthe first and second coils have been wound about the bobbin, aferrofluid 142 is injected into magnet cavity 140. The ferrofluid servesto at least partially fill the region of the magnet cavity 140 andspanning the radial gap between the outer periphery of magnet 136 andthe upper and lower bobbin. Ferrofluid 142 is injected into the magnetcavity 140 via a bore 144 formed in lower bobbin 132. A ball bearing 146shown in FIG. 16 and in enlarged view in FIG. 17 serves to seal bore 144and provide a bearing surface engaging the end of shaft 138. Asdescribed in the preceding paragraph, thrust disk 134 serves to axiallybias the magnet 136 and attract shaft 138 downwardly. This downwardbiasing force causes the shaft to engage ball bearing 146 and seal bore144. The ball 146 serves as a thrust bearing and a check valve enablingthe ferrofluid 142 to be injected subsequent to coil winding.

The advantage of ferrofluids as opposed to non-magnetic damping liquidsis that the fluid is attracted to the outer periphery of magnet 136.This prevents the fluid from leaking out and enables the gauge to bemanufactured with small amounts of fluid. As little as 0.02-0.05 ml offluid and preferably about 0.03 ml of ferrofluid is used to providedamping. The selected ferrofluid should have a viscosity measured at 27°C. of 1,000-50,000 Centipoise preferably 5,000-20,000 Centipoise.Additionally, the selected ferrofluid should have a relatively lowsaturation magnetization. Fluids having a saturation magnetization of200 gauss measured at 25° C. have proven to work satisfactorily.Preferably, the selected ferrofluid will have a saturation magnetizationat or below 200 gauss, preferably in the 100-200 gauss range.Ferrofluids meeting the above criteria are available from FerrofluidicsCorporation, 40 Simon Street, Nashua, N.H. Ferrofluids are generallydescribed in a booklet published by Ferrofluidics Corporation entitled“Ferrofluids: Physical Properties and Applications”, © 1986, which isincorporated by reference herein.

Magnet 136 is affixed to shaft 138 utilizing a mechanically deformedferrule 148. Magnet 136 is provided with a central axial bore which isapproximately two times the shaft diameter. Ferrule 148 is sized toslip-fit over shaft 138 and into the axial bore in magnet 136. The shaftand magnet are then positioned relative to one another in precisecoaxial alignment in an installation fixture and the ferrule 148 isaxially compressed. The ferrule is plastically deformed to cause theshaft and magnet to be securely affixed to one another. To furtherenhance the joint strength, the outer periphery of shaft 138 is knurledas previously described with reference to the second embodiment. Ferrule148 can be formed of a tin/silver or tin/antimony alloy, however, it isbelieved that zinc, copper, aluminum, or lead/tin alloys would likewiseperform satisfactorily.

An additional design detail illustrated in FIGS. 16 and 17 is ring 150formed in lower bobbin 132 and upper bobbin 135 for the purposes oflimiting the frictional side wall contact area between bobbins 132 and135 and shaft 138. By limiting the contact area of the bobbins 132 and135 which engage shaft 138, friction and the associated hysteresis canbe minimized.

The fourth gauge embodiment 160 is shown in FIG. 18. Gauge 160 issimilar to gauge 130 however, shaft 162 projects out of lower bobbin 164rather than upper bobbin 166. This alternative construction facilitatesthe mounting of gauge 160 on what is frequently referred to as the rearside of the printed circuit board 168 with the shaft 162 projectingthrough an aperture 170 formed on the printed circuit board. Asillustrated in FIG. 18 of 172, the lower bobbin 164 cooperates with afrusto-conical portion of aperture 172 precisely align gauge 160 andprinted circuit board 168. This gauge orientation is typical ofconventional cross-coil gauges used in automotive instrumentation. Thedisadvantage of this orientation however, is the difficulty in solderingthe gauge to the printed circuit board using a wave solder machine as aresult of the presence of shaft 162.

Another difference between gauge 160 and the embodiments previouslydescribed is bobbin ring 174 shown enlarged in FIG. 19. Bobbin ring 174serves to space upper and lower bobbins 166 and 164 apart and findtherebetween in a closed magnet cavity. Bobbin ring 164 is one of aplastic material and is provided with a series of internal longitudinalribs 176. Ribs 176 serve to enhance the damping forces caused by theferrofluid which partially fills the magnet cavity and spans the radialgap between the outer periphery of magnet 178 and ribs 176. This ringcan also facilitate different gauge lengths (magnets) without redesignor additional tools. The feature 176 of FIG. 19 can also be incorporatedin FIG. 16 when a separate ring is not used.

FIGS. 20 and 21 illustrate the two gauge mounting orientations. FIG. 20illustrates the orientation of gauge motor 62 as relative to printedcircuit board 180 graphic and light distribution layer 182 and pointer184. As shown, gauge motor 62 is mounted on the front side of printedcircuit board 180 along with other conventional electrical componentsenabling the gauge motor to be wave soldered onto the printed circuitboard. The very small size of gauge motor 62 of the present inventionresults in a small gauge footprint on a printed circuit board providingmore space for electronic components. This additional space can resultin a significant cost savings if the conductive plating on the printedcircuit board be only PC board 186 thus having to balance one anotherminimizing the bending load exerted on the motor printed circuit boardconnection resulting from vibratory loads.

A fifth gauge embodiment of 192 is shown in FIG. 22. This gaugeembodiment is generally similar to gauge 62 shown in FIG. 11. Theprimary difference being that flux ring 194 is trapped between shoulderson the upper bobbin 196 and printed circuit board 198.

It is also understood, of course, that while the form of the inventionherein shown and described constitutes a preferred embodiment of theinvention, it is not intended to illustrate all possible forms thereof.It should also be understood that the words used in the specificationare words of description rather than limitation and various changes maybe made without departing from the spirit and scope of the invention.

What is claimed is:
 1. A crossed coil gauge comprising: a shaft orientedalong a shaft axis; an oriented rare-earth magnet mounted to rotate withthe shaft along the shaft axis, the magnet supplying magnetic fluxdirected transverse to the shaft axis, the magnet shaped as a cylinderhaving a maximum radius measured from the shaft axis and an axiallength, wherein the length of the magnet exceeds the radius; a gaugehousing which houses the magnet within a magnet cavity, the gaugehousing providing support for rotation of the shaft and the magnet aboutthe shaft axis; a first coil and a second coil each wound around thegauge housing, wherein the first coil is wound generally perpendicularto the second coil and wherein the first coil and the second coilencircle the magnet; and a flux ring, disposed around the first coil andthe second coil and axially aligned with respect to the magnet and theshaft, said magnet having sufficiently high strength and said flux ringbeing mounted in sufficiently close proximity to the first and secondcoils to redistribute the magnetic flux supplied by the magnet such thatthe magnetic flux density near the first coil and the second coil isincreased by at least 30% in comparison to the magnetic flux densitysupplied by the magnet without the flux ring.
 2. The crossed coil gaugeof claim 1, wherein the flux ring comprises a cylindrical body adaptedto be mounted concentrically about the magnet, the cylindrical bodyhaving a uniform cross-section defined by an axial length and a uniformwall thickness.
 3. The crossed coil gauge of claim 2, wherein thecylindrical body has a wall thickness of at least 8% of the axial lengthof the body.
 4. The crossed coil gauge of claim 2, wherein thecylindrical body has a wall thickness of at least 10% of the axiallength of the body.
 5. The crossed coil gauge of claim 2, wherein thecylindrical body is formed of a fully dense powder metal alloy with lowresidual induction.
 6. The crossed coil gauge of claim 5, wherein thefully dense powder metal alloy is annealed to minimize circumferentialvariations in magnetic properties.
 7. The crossed coil gauge of claim 6,wherein the flux ring is mechanically sized subsequent to centering andprior to annealing to maintain a uniform inside diameter which varies nomore than 0.001 inches in roundness.
 8. The crossed coil gauge of claim7, wherein the cylindrical body has a uniform inside diameter whichvaries no more than 0.002 inches in roundness.
 9. The crossed coil gaugeof claim 5, wherein the flux ring is formed of fully dense nickel/ironalloy powder metal.
 10. The crossed coil gauge of claim 9, wherein thenickel/iron alloy powder metal is compacted to at least 7.0 g/cc priorto sintering.
 11. The crossed coil gauge of claim 10, wherein thenicker/iron alloy powder metal has a final sintered density of at least7.2 g/cc.